The Rise of Galvanic Skin Response (GSR) Sensors in Wearables

Maxim develops a wearable platform for better measurement of static activities such as yoga using GSR sensors.

The
wearable Galvanic Skin Response skin system, MAXREFDES73#, is a
wearable system recently released by Maxim. It's a mobile system that
uses the MAX32600 wellness measurement microcontroller to measure
impedance as well as temperature, making it an excellent fitness
tracking device.

Understanding the Galvanic Skin Response

The galvanic skin response (GSR) is also known as electrodermal
activity (EDA). These are terms that describe the continuous and
systemic variation of electrical properties of human skin. As GSR is not
fully understood, research into its causes and effects is ongoing.
In the past, GSR has been used to develop medical technology such as
the electrocardiograph (EKG). It's also been used as a method for
detecting psychological, emotional, and physiological arousal and
agitation by tracking sweat gland activity connected to sympathetic
activtity of the autonomic nervous system. In effect, GSR is capable of
indicating when someone is under stress. Because of this, GSR is one of
the metrics recorded in modern polygraph tests.

Sample of a GSR reading taken from a subject's middle and ring fingers. Image courtesy of Hugo Gamboa Dez via Wikimedia Commons.

Recently, more and more wearable devices have begun incorporating GSR technology. The Microsoft Band, for example, includes a GSR sensor
Another application of a wearable GSR sensor is the Moodmetric ring, which purports to track the wearer's emotional state using a GSR sensor.
Compared to this psychologically-focused approach, the MAXREFDES73#
differs in that it's being touted as a way to measure physical
activity.

Specs and Capabilities

One of its selling points is that it's sensitive enough that it can
detect the activity involved with yoga or other static exercises that
the more common fitness bands cannot detect.
How does it accomplish this? The GSR device uses an external negative
temperature coefficient sensor so that it can directly be in contact
with the skin for more precise measurements. The measurements are taken
every second and the measurement and transmitting time is about 30
milliseconds. During the rest of the time, the microcontroller is put
into a low power state to conserve power.
The wearable is battery powered and measures high-precision impedance
measurements using an ARM Cortex M3 32-bit RISC PCU. Its design
features the ARM microcontroller, Bluetooth for low power communication,
4 DACs, 1 ADC, embedded security, and multiple op amps.
The three LEDS (yellow, red, and green) give visual feedback
about the current state of the system, such as charging or taking
measurements.

The MAXREFDES73# comes with an Android mobile application
which requires Android version 4.3 or higher. It connects with the
device and displays the temperature and impedance magnitude. Using the
"Frequency Sweep" button on the application shows the Bode plot of the
latest data:

In addition to the GSR sensor, its features include the ability to
measure oxygen saturation, air temperature, wrist temperature, heart
rate, and position. It also gives more information about the overall
health and wellness of a person’s body and can determine the type of
exercise based on the measurements that it takes.
It does not currently have a casing to it but does have a band that can be strapped to the wrist.

Made for Wearables Developers

The MAXREFDES73# would be a great starting point for a new wearable project.
Its various health-monitoring features can help developers make
wearable devices quickly and with a relatively low amount of dollars
invested in making prototypes.

In addition, the schematic, PCB layout, and other design files are
included with the unit so implementing and combining other technologies
is a bit easier.

The reference design is flexible so it can be used in different
applications. Since it takes into account changes in temperature and
impedance more exercises can be monitored that may be missing from the
traditional heart rate and step monitors in most health wearables.

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